diff --git a/README.md b/README.md
index 9297ddb..36c6b98 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 `intake` is an arbitrary feed aggregator.
 
-## Feed Source Interface
+## Feed source interface
 
 The base `intake` directory is `$XDG_DATA_HOME/intake`. Each feed source's data is contained within a subdirectory of the base directory. The name of the feed source is the name of the subdirectory.
 
@@ -53,6 +53,25 @@ Anything written to `stderr` by the process will be logged by intake.
 
 The `fetch` action is used to fetch the current state of the feed source. It receives no input and should write feed items to `stdout` as JSON objects, each on one line. All other actions are taken in the context of a single item. These actions receive the item as a JSON object on the first line of `stdin`. The process should write the item back to `stdout` with any changes as a result of the action.
 
-An item must have a key under action` with that action's name to support executing that action for that item.
+An item must have a key under `action` with that action's name to support executing that action for that item. The value under that key may be any JSON structure used to manage the item-specific state.
 
 All encoding is done with UTF-8. If an item cannot be parsed or the exit code of the process is nonzero, intake will consider the action to be a failure. No items or other feed changes will happen as a result of a failed action, except for changes to `state` done by the action process.
+
+## Item fields
+
+An item has the following top-level fields:
+
+* `id`: **Required**. A unique identifier within the scope of the feed source.
+* `source`: **Automatic**. The source that generated the item. This attribute is automatically populated.
+* `created`: **Automatic**. The Unix timestamp at which the item was generated. This attribute is automatically populated.
+* `active`: **Automatic**. Whether the item is active. Inactive items are not displayed in channels.
+* `title`: The title of the item. If an item has no title, `is` is used as a fallback title.
+* `author`: An author name associated with the item.
+* `body`: Body text of the item as raw HTML. This will be displayed in the item without further processing.
+* `link`: A hyperlink associated with the item.
+* `time`: A time associated with the item, not necessarily when the item was created. Feeds sort by `time` when it is defined and fall back to `created`.
+* `tags`: A list of tags that describe the item. Tags help filter feeds that contain different kinds of content.
+* `tts`: The time-to-show of the item. An item with `tts` defined is hidden from channel feeds until the current time is past `created + tts`.
+* `ttl`: The time-to-live of the item. An item with `ttl` defined is not deleted by feed updates even if it is inactive if `created + ttl` is in the future.
+* `ttd`: The time-to-die of the item. An item with `ttd` defined is deleted by feed updates even if it is active if `created + ttd` is in the past.
+* `action`: An object with keys for all supported actions.